Explained: The Different Types Of Sustainable Aviation Fuel

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November 18, 2021 by Linnea Ahlgren

The term Sustainable Aviation Fuel is becoming a common catchphrase as the airline industry commits to net-zero within a few decades, and not without good reason. A pivot away from a reliance on conventional, fossil-based jet fuel is the single most crucial factor in decarbonizing aviation. Meanwhile, not all SAF is created equal. Let’s take a look at how the landscape is unfolding – today and tomorrow.

Understanding the SAF landscape

Traditional jet fuel has an overwhelmingly attractive energy density that allows operators to travel far without adding much weight – something fundamental to airline economics. Even when alternative propulsion technologies such as hydrogen and electric will enter the scene in the next decade, they will not solve the medium- and long-haul part of the emission equation for some time to come.

Enter SAF or Sustainable Aviation Fuel. It might not have the same effectful appeal as new types of airframes along the lines of Embraer’s Energia family or Airbus’ blended wing ZEROe concept. However, the development and scaling up of alternative fuels are crucial to mitigating aviation’s climate impact.

While one would be forgiven for thinking SAF is one specific thing, it is, in fact, a patchwork of technologies, all with their particular set of challenges. As Rohini Sengupta, Senior Manager of Environmental Sustainability and Climate for United Airlines, said during a presentation at an ACI Oleofuels online event this week,“Understanding the feedstock landscape and its potential to address the entire sector’s emissions is important, as it’s clear that SAF is going to be not only a short-term solution but a solution through 2050.”

The FOGs of today

Most biofuel available today comes from fats, oils, and greases – also referred to as FOGs. These not only serve the purpose of drop-in aviation fuel but also yield road biofuels such as renewable diesel. For instance, sustainable transport company Neste has a collaboration with McDonald’s in which they take used cooking oil from the fast-food chain’s 252 Dutch restaurants. This is then refined and turned into fuel.

FOGs are the first generation of biofuels. They reduce CO2 emissions compared to conventional jet fuel by 50% to 80% over the lifecycle of the product. Several technologies exist to separate FOGs from treatment and sewage centers. Still, the limitations on feedstocks will create a problem for significant scaling up of production for aviation supply purposes.

An increasing number of airlines have operated thousands of flights powered partially by FOGs. While several technologies are certified to produce SAF for use in commercial aviation, the products that are available today are mostly made through a process called HEFA. This stands for Hydrotreated Esters and Fatty Acids. The certification allows for a 50% blend, although most SAF-powered flights operate on far less.

Biomass and MSW

The next generation of Sustainable Aviation Fuels is derived from biomass and municipal solid waste (MSW). These have the potential to reduce greenhouse gas emissions by 85% to 95% over their lifecycle compared to traditional fossil-based jet fuel.

Biomass includes algae, crop residues, animal waste, sludge waste, and forestry residue. The fuel itself is derived from carbon-based materials contained in living organisms that can be gasified. One of the issues with biomass is that it has a low energy density compared to fossil fuels, meaning it requires a significantly larger volume to generate the same energy.

In 2015, United Airlines bought a $30 million stake in a company called Fulcrum BioEnergy. This allows the airline to purchase 90 million gallons per year for a period of ten years – once production picks up. However, just this year, United did one better with the largest publicly announced SAF agreement in history. The airline has committed to purchasing 1.5 billion gallons of SAF from a company called Alder Fuels – a company that says it will produce SAF on characteristic par with today’s jet fuel.

“To scale SAF as quickly as necessary, we need to look beyond existing solutions and invest in research and development for new pathways like the one Alder is developing,” Scott Kirby, United’s CEO, said at the time of the announcement in September.


Both first and second-generation biofuels are limited in their availability because of the nature of their feedstock. However, the most promising technology for SAF is only in its infancy.

Power-to-Liquid entails the conversion of renewable energy into fuels through electrolysis. Just earlier this month, an Ikarus C42 microlight aircraft belonging to the Royal Air Force operated a short flight from Cotswold Airport powered entirely by synthetic gasoline.

The fuel, UL91, is manufactured by a company called Zero Petroleum by extracting hydrogen from water and carbon from atmospheric carbon dioxide. The flight earned the RAF and Zero Petroleum a place in the Guinness Book of World Records.

Meanwhile, in February this year, KLM operated the world’s first passenger flight powered partially by synthetic kerosene in collaboration with Shell and the Dutch Ministry for Infrastructure and Water Management.

In the US, carbon transformation company Twelve recently announced the readiness of its first batch of commercially available E-Jet fuel with up to 90% less CO2 emissions over its lifespan compared to traditional jet fuel.


True circularity in the sky

Synthetic Power-to-Liquid fuels hold the promise to reduce greenhouse gas emissions by as much as 99%. If utilizing captured CO2 and green hydrogen, it can even create a carbon-neutral, circular system, picking up and using the CO2 that aviation is responsible for and turning it right back into power for the planes.

However, for this dream scenario to come true, there are a number of hurdles that need to be overcome. First of all, the carbon capture and storage industry needs to scale up beyond anything that current investment allows to provide feedstock. Secondly, there needs to be enough renewable energy due to the sheer amount of electricity the process requires. Meanwhile, if this is accomplished, there is, as we are all alarmingly aware, no scarcity of feedstock in terms of CO2 in the air.


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